The aims of this project are to: (1) localize the sites of change in pulmonary vascular resistance in the transition from the fetal to the neonatal state, and (2) investigate the factors which may control these changes. The effects of variation in alveolar pressure on pulmonary vascular resistance in the liquid-filled lung and the air-filled lung of exteriorized fetal goats will be studied using a model involving Starling resistors. By means of the model, changes in pulmonary vascular resistance will be segmented. Also sites of the postulated Starling resistors will be determined. Analysis of the specific locus of action of several pulmonary vasodilators and diuretics will be made in the fetal and neonatal state. The potential significance of this research lies in the fact that it will permit segmental analysis of normal and abnormal pulmonary circulation in the fetus and newborn. Knowledge of specific sites of pulmonary vascular resistance and factors which control this resistance should be of use in a rationale approach to therapy of respiratory distress.